1. Field of the Invention
This invention relates to a submersible motor electric wire (i.e., an electric wire for a submersible motor) capable of having a good water-treeing resistance for a long term and, in particular, to a submersible motor electric wire suitable for use under a radiation environment.
2. Description of the Related Art
A submersible motor electric wire is used for a coil (a winding wire) or the like of a submersible motor. The submersible motor electric wire is provided for the purpose of being used in water, as indicated by the name, so that it has an insulating sheath layer for covering a conductor. When the winding wire is immersed in water, it becomes in a state of containing excessive water between the molecules of the insulating sheath layer with the immersion time. If the insulating sheath layer containing water directly contacts with the conductor (for example, copper), copper ions deposit and diffuse into the insulating sheath layer from the surface of the conductor toward the periphery of the winding wire due to that voltage is applied at the operation of the submersible motor. Further, due to the copper ions, a water-treeing occurs in the insulating sheath layer, and the water-treeing causes an insulation deterioration or an insulation breakdown of the winding wire.
Therefore, conventionally, the submersible motor electric wire is configured to have a conductor shielding insulating layer (for example, an enamel layer made of an enamel resin) formed on the periphery of the conductor as a film for preventing the deposition and diffusion of the copper ions (for example, refer to JP-A-05-325653).
On the other hand, cross-linked polyethylene can be used as a material of the insulating sheath layer of the submersible motor electric wire in order to improve heat resistance.
The cross-linked polyethylene is obtained by conducting cross-linking between the molecules of polyethylene so as to form a reticular molecule structure, and various methods have been used as the cross-linking method, but in case of thermal chemistry cross-linking method it is preferable to carry out the cross-linking by adding an organic peroxide as a cross-linking agent such as dicumyl peroxide (as expressed by chemical formula (A)) into polyethylene and conducting heat treatment so as to initiate a reaction.
As a result, cross-linking cracked residues produced incident to the above-mentioned chemical reaction may exist in the cross-linked polyethylene corresponding to the insulating sheath layer after cross-linking. The cross-linking cracked residue may be, for example, cumyl alcohol (as expressed by chemical formula (B)), acetophenone (as expressed by chemical formula (C)), and α-methylstyrene (as expressed by chemical formula (D)).

The above related arts are disclosed in, e.g., JP-A-61-114410 and JP-A-04-87222.
Further, at present day, there is a tendency that the submersible motor electric wire is needed to be increased in longevity, so that it is needed to further enhance the water-treeing resistance. “To enhance the water-treeing resistance”, which may be also expressed “to enhance a water-treeing property”, means “to prevent the occurrence of water-treeing”.
As mentioned above, if the submersible motor electric wire is immersed in water in a state that the insulating sheath layer made of the cross-linked polyethylene or the like directly contacts with the conductor, copper ions deposit and diffuse into the insulating sheath layer, and due to the copper ions, a water-treeing occurs and grows in the insulating sheath layer, so as to causes a short-time insulation breakdown of the winding wire. As a measure of this, the enamel layer is disposed between the conductor and the insulating sheath layer as the conductor shielding insulating layer.
However, the submersible motor electric wire configured to have a structure that the polyethylene of the insulating sheath layer is cross-linked by the thermal chemistry cross-linking method and the enamel is used as the conductor shielding insulating layer may affect the water-treeing resistance thereof, since if it is used for a long time, the enamel swells due to that the cross-linking cracked residue penetrates between the molecules of the enamel resin so that a crazing occurs in the enamel resin. This is due to that the cross-linking cracked residue has a benzene ring (a kind of aromatic ring) and has a low molecular weight so that it easily penetrates between the molecules of the enamel resin which has a benzene ring (a kind of aromatic ring) as well as the residue.